1. Field of the Invention
The present invention relates to a motor vehicle automatic transmission and more particularly to a control of a motor vehicle automatic transmission. More specifically, the present invention pertains to a hydraulic control system of an automatic transmission having a multiple-stage transmission gear mechanism having three or more gear stages.
2. Description of Prior Art
Conventional automatic transmissions for motor vehicles include a torque converter and a multiple-stage transmission gear mechanism connected with the torque converter to be driven thereby. The gear mechanism generally has three forward drive stages and one reverse drive stage. In recent automatic transmissions such as the one disclosed by the published U.K. patent application No. 2,057,602, there is arranged an overdrive gear mechanism between the torque converter and the transmission gear mechanism so that an overdrive gear stage is additionally provided. The transmission is provided with a hydraulic control system for selecting an appropriate one of the gear stages in accordance with the vehicle operating conditions, such as the engine output and the vehicle speed. There has also been proposed to provide a multiple stage gear transmission which can provide smooth gear shift operations without having a hydraulic torque converter. This type of transmission may be advantageous in respect of efficiency, over a transmission having a torque convertor.
As one of control modes performed by the control system, there is provided a kick-down control wherein the gear mechanism is shifted down to a lower gear stage when an engine control member such as an engine throttle valve actuating member is moved to a substantially full power position. Where the automatic transmission has such kick-down control mode, and the hydraulic control system is of such type that the gear shift control valves in the control system are operated by hydraulic pressure signals representing the engine operating condition, there is provided a kick-down valve for applying a hydraulic pressure to each gear shift control valve to bias the gear shift valve in a gear shift down sense. More specifically, in a conventional hydraulic control system of the aforementioned type, the shift valve has an axially slidable spool which is applied at one end with a governor pressure representing the vehicle speed in a direction of forcing the spool in the shift up sense and at the other end with a throttle pressure, representing the engine output in a direction of forcing the spool in the shift down sense. The hydraulic pressure from the kick-down valve is applied in the direction of assisting the throttle pressure so that the shift down takes place at a speed higher than the speed under which the shift down normally takes place. An example of such a hydraulic control system is shown, for example, in Japanese patent disclosure No. 58-137650.
The hydraulic control circuit disclosed in the Japanese patent disclosure publication is for the control of four-stage gear transmission mechanism having a three-stage gear mechanism connected with an over-drive gear mechanism so that the circuit has three shift valves, one for the control of the shifting operation between the first and second gear stages, the second for the control between the second and third gear stages and the third for the control between the third and fourth gear stages. In the hydraulic control circuit disclosed by the Japanese patent disclosure publication, the hydraulic pressure from the kick-down valve is applied simultaneously to the three shift valves. It will therefore be understood that there is a possibility that the transmission mechanism be shifted down from the fourth stage directly to the second stage without passing through the third stage, or under some circumstances, it may be shifted down from the fourth stage once to the second stage and then shifted up to the third stage. These operations are undesirable because there will be produced uncomfortable shifting shocks.